Module documentation for 0.1.1.0

It's iternally implemented by Data.IntMap.Strict.IntMap or Data.Map.Strict.MapInteger,
using Int or Integer as the iterator type respectly.
Most of the operations cost O(lg N).

Each newly inserted element will consume a unique number and never reuse old numbers.
Choose Int one if you're sure that there're no more than Int space times of insertions,
or choose Integer one otherwise.